1. Field of the Invention
The present invention relates to a two-shaft gas turbine and a method for starting premixed combustion in a combustor for a two-shaft gas turbine.
2. Description of the Related Art
A general two-shaft gas turbine has a gas generator and a low-pressure turbine (power turbine). The gas generator includes a compressor, a combustor and a high-pressure turbine. The low-pressure turbine is connected to a load device. The gas generator has a rotary shaft separated from a rotary shaft of the low-pressure turbine.
The compressor included in the gas generator compresses air. In the combustor, the compressed air and fuel are mixed and burned to form a combustion gas.
After the high-pressure turbine is driven to rotate and thereby generate a drive force for the compressor, the combustion gas formed in the combustor is transferred to the low-pressure turbine. Then, the low-pressure turbine is driven to rotate.
In the conventional combustor included in the gas turbine, both diffusion combustion and premixed combustion are carried out. In the diffusion combustion, fuel and air are burned while being mixed with each other. In the premixed combustion, fuel and air are premixed and then burned. The diffusion combustion is carried out until a load is increased from zero to a certain level. The premixed combustion starts when the load reaches the certain level. This combustion process stabilizes a flame in the combustor and suppresses generation of a nitrogen oxide (NOx).
JP-07-280267-A discloses a combustor including a burner for diffuse combustion and a burner for premixed combustion. The burner for premixed combustion is arranged around the burner for diffusion combustion. The burner for premixed combustion includes a plurality of regions arranged in a circumferential direction of the burner for diffusion combustion. In the combustor, the premixed combustion can be carried out separately in each of the regions.
In the combustor described in JP-07-280267-A, the diffusion combustion is carried out until a load is increased from zero to a certain level. Then, a flame is stabilized in the combustor. After that, the premixed combustion starts on the basis of the amount of fuel that is to be introduced into the combustor. The amount of the fuel is increased with the increase in the load. A region in which the premixed combustion is to be carried out is selected from the plurality of regions on the basis of the amount of the fuel. The premixed combustion starts separately in each selected region. The number of the selected regions in which the premixed combustion is carried out is increased with the increase in the amount of the fuel that is to be introduced into the combustor. This method suppresses generation of a nitrogen oxide.
A region in which the premixed combustion is to be carried out is selected from the plurality of regions on the basis of the amount of the fuel. The number of the selected regions in which the premixed combustion is carried out is increased with the increase in the amount of the fuel that is to be introduced into the combustor.